The Milky Way is home to hundreds of billions of stars, and many more planets. Some come in sets, as in our own solar system. But not every planet orbits a star.
Some planets actually wander the galaxy alone, untethered. They have no days or nights, and they exist in perpetual darkness. In a kitschy NASA collection of travel posters for destinations beyond Earth, one of these cold worlds is advertised with the motto: “Visit the planet with no star, where the nightlife never ends.”
Astronomers call these worlds free-floating, or rogue, planets. They are mysterious objects, and a small group of researchers around the world is dedicated to studying them. Of the thousands of planets that scientists have detected beyond our solar system so far, only about a dozen are sunless and coasting on their own, somewhere between us and the center of the Milky Way. At least, astronomers think they are. “We are sure that these objects are planets,” Przemek Mroz, an astronomer at Caltech, told me. “We are not fully sure whether these objects are free-floating or not.”
Mroz has spent perhaps as much time thinking about these strange objects as anyone on Earth. He and his team just announced another finding—the smallest known rogue planet—today. The object is between the masses of Earth and Mars, a blip in interstellar space so relatively tiny that it might seem insignificant. But according to scientists’ best theories about the way planetary systems arise all across the universe, rogue worlds should exist.
The term rogue planet suggests that these objects desert their stars on purpose, striking out on their own to carve a new path through the Milky Way. In reality, rogue planets are usually kicked out of their star system, banished to a solitary existence circling the center of the galaxy.
The beginnings of a planetary system, including our own, are thought to be quite messy. As planets swirl into shape out of the cosmic fog surrounding a newborn star, they jostle one another around. The gravitational game of pool can shove planets toward the edges of a system, and even eject them altogether. Nearby stars can scramble planets too. Most stars are not born alone, but in clusters of dozens to thousands, and in such a crowded environment, a passing star with its own entourage of planets could whisk away a planet from another, keeping it for itself or casting it out into space.
Some solitary planets might form another way, without the help of a parent star. These worlds emerge from collapsed clouds of gas and dust, as stars do, but they don’t have enough mass to spark the nuclear reactions that make stars shine. These objects, known as “failed stars”—wow, astronomers—resemble planets from afar.
Rogue planets are extremely difficult to detect; astronomers can’t search for them like they do exoplanets, which reveal their presence by gently tugging at their parent stars or briefly blocking out their light as they go around. On the loose and nearly invisible, rogue planets evade detection in much the same way that black holes do.
So astronomers rely on a cosmic quirk of gravity. Imagine a line of sight from Earth’s telescopes to a distant star. When an object crosses that line, its presence can bend and magnify the star’s light, making the star appear more luminous than usual to us. The duration of the brightening signals the nature of the object responsible—a brightening that lasts several days indicates a star, a day means a Jupiter-mass object, and hours suggest something equaling the mass of Earth. The rogue planet recently discovered by Mroz’s team signaled its existence for just a few hours.
The tricky part is figuring out whether rogue planets are, in fact, rogue. The stars whose light they bend can’t be their parent stars because they’re simply too far away. And even if a parent star were closer by, it would be impossible to see through the luminous star’s glare. Astronomers must wait years, usually a decade, for the luminous star to move before they can check for a parent star. If no such star appears, the planet is probably going solo. The process takes long enough that scientists haven’t reached this milestone for any of the dozen rogue-planet candidates, including the latest, tiniest addition.
Mroz and other astronomers studying rogue planets don’t know how many of these worlds might be coasting through the Milky Way, nor do they know much about the ones they’ve found so far. They can discern the mass of an object through their observation and compare it with worlds in our own solar system—objects with masses similar to those of Earth and Mars, for example, are probably rocky, while objects as massive as Neptune and Uranus are icy. But those analogies cannot fill in the details of rogue planets’ unknown surfaces, or the atmospheres that separate them from space.
There’s no doubt about one thing: Without a star to warm themselves by, rogue planets must be frozen—if not to their core, certainly at their outermost layer. They might not be so alone, either; planets could take their moons with them when they’re hurled out of their cosmic homes.
As they roam through the galaxy, what can happen to rogue planets? Could a free-floating world find a home out there with a different star? Michael Liu, an astronomer at the University of Hawaii, thinks it’s unlikely. Interstellar space is quite, well, spacious, and it’s difficult for even a hefty star to slow down and lasso a fast-moving planet. In 2017, an interstellar asteroid the size of a skyscraper barreled right through our solar system and just kept going. “Normally, things just whiz by each other,” Liu says.
Could something bigger—an entire rogue planet—catch us by surprise as that asteroid did? The answer to this unnerving question depends on how common rogue planets are. “Do I worry about a free-floating planet hitting the solar system? No, but maybe I should?” says Jennifer Yee, an astrophysicist at Harvard and Smithsonian’s Center for Astrophysics who uses the same line-of-sight technique to find exoplanets. “It really depends on how many there are. If there are one per star, it isn’t very likely that we would run into one.”
A surprise visit from a rogue planet would present astronomers with a great research opportunity. It would also likely terrify the rest of us. “Probably we would be fine because the solar system itself is pretty empty,” Yee says. “On the other hand, depending on how massive the planet is, it might perturb the orbits of the existing planets, which could be bad.”
The orbits of our planets will someday become perturbed anyway. About 5 billion years from now, our sun—that glowing, life-giving, seemingly immutable orb—will start to die. The star will lose mass until it can no longer hold onto its outermost planets. Neptune and Uranus—and Pluto too—will probably become rogue planets. They will drift away, taking their icy atmospheres with them. Unbothered by the cold of interstellar space, the planets will remain mostly unchanged, relics of a solar system that once huddled close around a warm sun.
Earth will meet a different fate. Dying stars lose mass because they eject gas and dust in all directions, leaving exposed their spent cores. Our planet is expected to become enveloped in this hot mist and vaporized.
For now, Earth remains safely tucked into the solar system, on a cozy orbit from which we can look out at other, lonelier worlds. Astronomers are eager for the launch of a new telescope scheduled for the mid-2020s. The Nancy Grace Roman Space Telescope—named for NASA’s first female executive, recognized for helping make the agency’s best-known space telescope, the Hubble, a reality—will have an exquisite view of the night sky. Free from the atmosphere that often stymies ground telescopes, Roman, as the telescope is called, will peer toward the heart of the Milky Way, crowded with stars. A recent study predicted that Roman could detect hundreds of rogue planets, and would provide the best estimate for these worlds yet. Right now, estimates range from tens of billions to trillions.
Roman might find fewer true rogue planets than astronomers expect, or perhaps none at all. Their obsession could turn out to be a very minor footnote in the galaxy’s story. Or it could change our understanding of the place we live. Sam Johnson, a graduate astronomy student at the Ohio State University and the lead author of the Roman study, likes to imagine himself on one of these worlds, blanketed in pure darkness, the rest of the Milky Way stretching out in front of him. “They can feel pretty lonely, I would imagine,” Johnson says.